A personal union is the combination of two or more states that have the same monarch while their boundaries, laws, and interests remain distinct.[1] A real union, by contrast, would involve the constituent states being to some extent interlinked, such as by sharing some limited governmental institutions. In a federation and a unitary state, a central (federal) government spanning all member states exists, with the degree of self-governance distinguishing the two. The ruler in a personal union does not need to be a hereditary monarch.[2]

The term was coined by German jurist Johann Stephan Pütter, introducing it into Elementa iuris publici germanici (Elements of German Public Law) of 1760.[3]

Personal unions can arise for several reasons, ranging from coincidence (a woman who is already married to a king becomes queen regnant, and their child inherits the crown of both countries; the King of one country inherits the crown of another country) to virtual annexation (where a personal union sometimes was seen as a means of preventing uprisings). They can also be codified (i.e., the constitutions of the states clearly express that they shall share the same person as head of state) or non-codified, in which case they can easily be broken (e.g., by the death of the monarch when the two states have different succession laws).

Because presidents of republics are ordinarily chosen from within the citizens of the state in question, the concept of personal union has almost never crossed over from monarchies into republics, with the rare exception of the president of France being a co-prince of Andorra. In 1860 Marthinus Wessel Pretorius was simultaneously elected as the president of Transvaal and Orange Free State and he tried to unify the two countries but his mission failed and led to the Transvaal Civil War.

Congo Free State and Belgium

Personal union with Belgium from 1885 to 1908, when the Congo Free State became a Belgian colony. The only sovereign during this period was Leopold II, who continued as king of Belgium until his death a year later in 1909.

Note: The point at issue in the War of the Spanish Succession was the fear that the succession to the Spanish throne dictated by Spanish law, which would devolve on Louis, le Grand Dauphin — already heir to the throne of France — would create a personal union that would upset the European balance of power; France had the most powerful military in Europe at the time, and Spain the largest empire.

Hungary

Personal union with Poland from 1370 to 1382 under the reign of Louis the Great. This period in Polish history is sometimes known as the Andegawen Poland. Louis inherited the Polish throne from his maternal uncle Casimir III. After Louis' death the Polish nobles (the szlachta) decided to end the personal union, since they did not want to be governed from Hungary, and chose Louis' younger daughter Jadwiga as their new ruler, while Hungary was inherited by his elder daughter Mary. Personal union with Poland for the second time from 1440 to 1444.

Personal union with Naples from 1385-1386 under the reign of Charles III of Naples.

Personal union with Bohemia, 1419–1439 and 1490–1918.

Personal union with the Archduchy of Austria, 1437-1439, 1444-1457, and 1526-1806.

The King Chungseon reigned as King of Goryeo in 1298 and 1308–1313 and King of Shenyang or Shen from 1307 (according to the History of Yuan) or 1308 (according to Goryeosa) to 1316. At that time, Goryeo had already become a vassal of Yuan and the imperial family of Yuan and the royal family of Goryeo had close relationship by marriages of convenience. Because he was a very powerful man during Emperor Wuzong's era, he could become the King of Shenyang where many Korean people lived in China. However, he lost his power in the court of Yuan after the death of Emperor Wuzong. Because the Yuan Dynasty made Chungseon abdicate the crown of the Goryeo in 1313, the personal union was ended. King Chungsuk, Chungseon's eldest son, became the new King of Goryeo. In 1316, the Yuan Dynasty made Chungseon abdicate the crown of Shen in favour of Wang Go, one of his nephews, resulting in him becoming the new King of Shen.

Navarre

Personal union with France from 1285 to 1328 due to the marriage between Philip IV of France and Joan I of Navarre and the reign of their three sons, and from 1589 to 1620 due to the accession of Henry IV, after which Navarre was formally integrated into France.

Sardinia

Saxe-Coburg and Saxe-Gotha

In 1826, the newly created Duchy of Saxe-Coburg and Gotha was initially a double duchy, ruled by Duke Ernest I in a personal union. In 1852, the duchies were bound in a political and real union. They were then a quasi-federal unitary state, even though later attempts to merge the duchies failed.

Schleswig and Holstein

The kings of Denmark at the same time being dukes of Schleswig and Holstein 1460–1864. (Holstein being part of the Holy Roman Empire, while Schleswig was a part of Denmark). The situation was complicated by the fact that for some time, the Duchies were divided among collateral branches of the House of Oldenburg (the ruling House in Denmark and Schleswig-Holstein). Besides the "main" Duchy of Schleswig-Holstein-Glückstadt, ruled by the Kings of Denmark, there were states encompassing territory in both Duchies. Notably the Dukes of Schleswig-Holstein-Gottorp and the subordinate Dukes of Schleswig-Holstein-Beck, Schleswig-Holstein-Sonderburg-Augustenburg and Schleswig-Holstein-Sonderburg-Glücksburg.